1. Field of the Invention
The invention generally relates to an obstacle detection device for a vehicle, which is mounted in the vehicle, to help a driver become aware of obstacles and the like. More specifically, the invention relates to an obstacle detection device for a vehicle which minimizes the false recognition of objects that cause virtual images in a captured image.
2. Description of the Related Art
In recent years, the use of CCD cameras and millimeter wave radar has become widespread in order to help a driver become aware of obstacles and the like while driving. Typically, CCD cameras have the benefit of being able to detect lane markings and lights and the like on the road, but are inferior in terms of accuracy with far distances because they calculate distance using a captured image. On the other hand, millimeter wave radar has the benefit of being extremely accurate with distances, but can only detect objects that have a large reflection coefficient.
As a related obstacle detection device for a vehicle, an obstacle detection device for a vehicle such as that disclosed in Japanese Patent Laid-Open Publication No. 8-156723, for example, has been proposed. This obstacle detection device for a vehicle compares the distance to an object in front of the vehicle calculated from an image captured by a CCD camera with the distance to the object ranged by the millimeter wave radar, and then recognizes the object as an obstacle when the distances match. This obstacle detection device for a vehicle makes use of the strong points of the CCD camera and the millimeter wave radar to make up for their weak points, thus enabling reliable recognition of an obstacle.
This type of obstacle detection device for a vehicle compares the calculation results from the captured image with the measurement results from the millimeter wave radar at a predetermined interval of time (e.g., every 1s). When the results match such that a highly dangerous situation is detected, a warning is issued and the user is alerted. To prevent a collision, the user is alerted the instant that both the results match up and the obstacle enters an area where the danger of collision is high.
Even with an obstacle detection device for a vehicle that uses both a CCD camera and millimeter wave radar, however, there is a possibility that an object, with which there is no danger of collision, may be erroneously recognized as an obstacle due to a virtual image appearing on the image captured by the CCD camera.
More specifically, when there is a puddle on the road, or when the road surface on which the vehicle is running tends to reflect light, such as the road itself at night, a virtual image created by the reflection of the road surface may be captured on the image that is captured by the CCD camera. This virtual image is not a real object, and therefore will not be detected by the millimeter wave radar.
When there is a metal object such as a can, or a road joint or an uneven portion of the road, or a piece of metal in the same location as this virtual image, however, these are ranged by the millimeter wave radar. At this time, the calculation results from the captured image and the measurement results from the millimeter wave radar are consistent, so the obstacle detection device for a vehicle may recognize these metal objects as obstacles and issue an erroneous warning.
This type of erroneous warning may cause the user to distrust the obstacle detection device for a vehicle, thereby hurting the merchantability of the obstacle detection device for a vehicle.